Malate Metabolism intheDarkAfter 13C02 Fixation inthe Crassulacean Plant Kalanchoe tubiflora

Themetabolism of[13C]malate wasstudied intheCrassulacean plant Kalanchoe tubiflora following exposure to13CO2 for2hour intervals during a16hourdarkcycle. Nuclear magnetic resonance spectroscopy of[13C]malate extracted fromlabeled tissue revealedthatthetransient fluxofmalate tothemitochondria, estimated bytherandomization of[4-13C]malate to[1-13C]malate byfumarase, varied substantially during thedarkperiod. Atboth 15and250C, theextent ofmalatelabel randomization inthe mitochondria wasgreatest during theearly andlate parts ofthe darkperiod andwasleast during themiddle ofthenight, when therateof13CO2 uptake washighest. Randomization oflabeled malatecontinued formanyhoursafter malatesynthesis had initially occurred. Internally respired 12CO2 alsoserved asasource ofcarbon formalate formation. At150C, 15%ofthetotal malate wasformedfromrespired 12CO2, whileat250C, 49%ofthe accumulated malate wasderived fromrespired 12CO2. Someof themalate synthesized fromexternal 13CO2 wasalsorespired during thenight. Theproportion ofthetotal [13CJmalate respired during thedarkperiod wassimilar at15and250C, andrespiration ofnewlyformed[13CJmalate increased asthenight period progressed. Thesedataarediscussed withregard totherelative fluxes ofmalate tothemitochondria andthevacuole during dark CO2fixation.